The present invention relates to a magnetic disk storage apparatus, and more particularly, to a servo system which mechanically drives transducers in the magnetic disk storage system. Specifically, the present invention relates to a servo positioning system for use with closed-loop disk drive systems using a dedicated servo surface.
Magnetic disk storage systems, particularly those which use Winchester technology, and are thus known as "Winchester disk drive systems," are comprised of a stack of magnetic disks, a plurality of transducers, each of which cooperates electromagnetically with the corresponding magnetic disk, a motor which moves the transducers with respect to the magnetic disks, and circuitry for controlling the motor such that the transducers are moved to a desired position on the magnetic disks in accordance with a command from a central processing unit. In the case of closed-loop positioning systems utilized in such Winchester disk drive systems, the transducers are typically moved by voice coil motors. Various data may be reached on at least one of the magnetic disks, for use by the controller to ascertain the position of the transducers on the disk surface.
In some closed-loop positioning system Winchester disk drives, one surface of one of the magnetic disks is usually utilized as a servo disk, while the others are utilized as data disks. For example, in concurrently filed U.S. patent application Ser. No. 854,949, entitled "Rotary Voice Coil Actuator Micro-Hard Disk Drive System," commonly assigned herewith, the disclosure of which is incorporated herein, a maximum of four magnetic disks may be provided. The top surface of the top disk, in the preferred embodiment, is designated as the servo disk and contains servo positioning information. The remaining disks are utilized for data storage. One prior art magnetic disk storage apparatus having a dedicated servo disk and several data disk surfaces is disclosed in U.S. Pat. No. 4,068,267, to Inouye, entitled "Magnetic Disk Storage Apparatus in Which Servo Track Zone Provides Signals for Both Moving Speed and Position of Transducer."
In the disk drive system discolosed in a U.S. patent application entitled "Rotary Voice Coil Actuator Micro-Hard Disk Drive System," the servo data contained on the servo disk is utilized to control the positioning of the transducers, under servo control. This system is known as a pure closed-loop type positioning system. In the patent to Inouye, a dedicated servo disk is also used.
There is also known in the art a quasi-closed-loop disk drive system in which a dedicated servo surface is utilized in conjunction with a number of data surfaces which carry both data and servo information. That servo information is usually recorded in sectors around the circumference of the data disks, interspersed with the data. Examples of such prior art systems are U.S. Pat. No. 3,944,016, to Moghadam, entitled "Positioning Servo System for Disk Drives" and U.S. Pat. No. 4,072,990, to Case et al, entitled "Servo Positioning Ssytem for Data Storage Apparatus." Such systems have the disadvantage that additional data space is lost on the various data disks, in order to provide servo information in addition to that recorded on the dedicated servo surface.
Another approach taken in the prior art is set forth in U.S. Pat. No. Re. 32,075 entitled "Data Transducer Position Control System for Rotating Disk Data Storage Equipment," to Harrison et al, in which a single servo sector is located on one surface of one of the disks. A coarse positioning system, having a light grating, is utilized for coarse positioning while the servo sector is utilized for fine positioning. Likewise, in the Case et al and Moghadam devices, the dedicated servo surface is utilized for fine positioning while a different system is utilized for coarse positioning.
All of the above-mentioned prior art devices utilize a phase-locked-loop or oscillator in controlling the transducer positioning. Also, with respect to those disk drive systems which utilize portions of each disk surface in addition to a dedicated servo surface, there is necessarily less usable data recording area and thus a lower capacity disk drive.
The elimination of such phase-locked-loop or oscillator, as applicant has to accomplished, renders the servo positioning mechanism much more immune to drift as well as eliminating the need for offsets and adjustments to the phase-locked-loop itself. The servo positioning system of the present invention allows the production of a disk drive system of high capacity and accuracy. In addition, no "out-of-lock faults" can occur due to defects in the media of the disks.